Transmissions of large industrial vehicles are usually divided into a main transmission, a splitter group transmission and a range transmission. At the same time, these transmissions have a shifting device supported by pneumatic circuit conducts pneumatic gearshifts in the splitter group transmission and in the range transmission while, at the same time, the gearshifts in the main transmission are manually carried out by the driver being supported by the pneumatic circuit in the force to be exerted.
Electronic devices are used to a greater extent for control of motor vehicle transmissions used today. These arrangements serve, together with other ends, also prevent, in the transmission, gearshifts that are not admissible due to other conditions, in particular to high vehicle speeds calculated on the gear intended to be shifted.
Such a pneumatic shift device with mechanisms for protecting against incorrect actuation has become known from DE-OS 24 47 860. The described device has different elements dispersed throughout the whole transmission so that, between the individual pneumatic elements, complicated branching air pressure lines are needed and a multiplicity of electric lines of individual elements for the electronic control device have to be laid. The parts comprise a main shut-off valve for the pneumatics and a gate blocking cylinder.
The pneumatic switch of a group transmission is triggered by a pneumatic valve on the shift lever. In certain cases, the result of this is that the driver, despite an activated gate block, can select the incorrect gate since the pressure build up in the gate block cylinder occurs too slowly due to the line lengths.
Besides, German patent applications DE 100 29 497 A1 and DE 100 29 527 A1 have disclosed an electro-pneumatic shift unit for a vehicle transmission having a pneumatically shifted range group transmission whose gearshifts can be preselected manually on a shift lever by the driver. The shift unit comprises a blocking device for mechanically preventing the manual gearshift of inadmissible gear ratios of the vehicle transmission and a device for preventing the pneumatic gearshift of inadmissible gear ratios of the vehicle transmission. The blocking device for mechanically preventing the manual gearshift and the device for preventing the pneumatic gearshift are controlled by a common control device.
The shift unit is now designed so as to result in preventing an undesired direct change from the shift gate for the gears five and six of the gear group of high gears to the shift gate lying in the same location for the gears one and two of the gear group of lower gears. For the purpose, in a preselected change from the gear group for higher gears to the gear group for lower gears, the shift shaft is then automatically moved by the blocking device by way of a recoil force produced in the shift gate of the gears three and four when the transmission shift lever, after the range group change preselection, has been brought to its neutral position.
A ramp built in the shift shaft and axially extending there with constant axial gradient services, during interaction with a pin of the blocking device engaging there, for the purpose that in the operating situation described, the shift shaft is autonomously pushed with constant force to the gate for the third and fourth gears. The blocking device and particularly the ramp in the shift shaft are constructed and synchronized with each other so that the driver, when needed, can overpress the recoil force applied by the blocking device together with the ramp. Thereby a downshift is determined by need with gear group change, for example, from the fifth gear to the second gear.
Although the already known electro-pneumatic shift unit has a few advantages, it is of comparatively complex construction. Besides, the shift unit is associated with a technical problem that the blocking force with which the shift shaft is held in the gate for the third and fourth gears must be relatively great in order that the driver, via the increased control force to be applied by him in a shift lever actuation, is reliably signalized that a group change has taken place when an incorrect gearshift is to be prevented.
In a transmission shift device, between the shift lever and the connecting means of the transmission, since there are some parts to be moved, their bearing friction on other parts caused by motion, the same as the gate blocking force, take care in the extreme case that despite the control force of the driver on the shift lever, the gate blocking described cannot be overpressed.
Therefore, the instant invention is based on the problem of providing a shift device for a generic transmission which is of simple construction and reliably prevents incorrect gearshifts between ratio groups of the range group transmission by way of a blocking device, the same as makes possible an emergency downshift across the range group of the transmission from a very high gear to a very low gear.